As interest-based advertising, sometimes called behavioral advertising, has spread, so has interest in some form of Do Not Track mechanism inspired by the federal Do Not Call Registry. Developers have added tools to Chrome, Firefox, and Safari to implement different versions of Do Not Track.

Last December, the FTC released a 122-page staff report on Do Not Track that called for an opt-out path for third-party behavioral advertising, meaning it wouldn't affect Web sites that compile their own user profiles. It stopped short, however, of calling for mandatory regulations--and the two Republican commissioners expressed concerns about its feasibility at the time.

CNET asked Brill whether, in light of the technological advances in the last 10 months, she would push for a law making some sort of do not track mechanism mandatory.

"A tremendous amount has been done," she replied. "Have all of the criteria been met? ... I'd say that we're not quite there yet. We're getting there. As I said, there are pluses and minuses to all the different mechanisms.... It's still a work in progress."

In August, Republican FTC commissioner J. Thomas Rosch laid out a different approach that that would involve investigating online advertising, requiring all ad networks to disclose what they do, and then perhaps regulating the industry. "We should have reliable information of that kind before we proceed further, as policymakers, and accuracy is much more important than speed," Rosch said.

Last month, the W3C standards body created a Tracking Protection Working Group, which aims to publish standards by mid-2012.

One possibility is that if Do Not Track becomes widely adopted, advertisers will not be able to target ads as precisely and some Web sites may no longer be able to survive as free services. If that happens, Web sites could start charging for access by requiring credit cards and real names--not the most privacy-sensitive result.

"That is certainly not an intent or something we are aiming to do," Brill said when asked about that possibility. "I don't think we're going to see a vast exodus of consumers from current Web sites or opting out of the way their information is used."

About the author

Declan McCullagh is the chief political correspondent for CNET. You can e-mail him or follow him on Twitter as declanm. Declan previously was a reporter for Time and the Washington bureau chief for Wired and wrote the Taking Liberties section and Other People's Money column for CBS News' Web site.
See full bio